Deep-Ultraviolet Nonlinear Optical Crystal Cs 2 Al 2 (B 3 O 6 ) 2 O: A Benign Member of the Sr 2 Be 2 (BO 3 ) 2 O Family with [Al 2 (B 3 O 6 ) 2 O] 2- Double Layers.

For the explorations of deep ultraviolet (DUV) nonlinear optical (NLO) borates, a type of important optoelectronic material, the (BO3 )3- group has been long regarded as the sole microscopic optically-active unit, and toxic Be-containing raw materials are frequently-adopted. Herein, a new DUV NLO crystal, Cs2 Al2 (B3 O6 )2 O (CABO), was designed and synthesized by simultaneously replacing the (BO3 )3- groups and Be2+ cations for (B3 O6 )3- units and Al3+ cations in Sr2 Be2 (BO3 )2 O, which possesses a favorable structure, through a chemical co-substitution approach. CABO exhibits a considerable DUV NLO capability because of the wide band gap and large birefringence originating from the [Al2 (B3 O6 )2 O]2- double layers. Remarkably, CABO melts congruently and does not contain the toxic beryllium, which is favorable for bulk-size crystal growth and practical applications.